Miniature receivers, such as cellular telephones are well known in the art. While those receivers have advanced to a sophisticated level, there are still a number of shortcomings.
One particular shortcoming arises in the miniaturization of such a receiver. Downsizing a radio receiver has proven to be problematic in that the conventional components of a radio receiver are typically bulky components relative to the overall size of the miniature radio receiver and are not readily adaptable to be implemented in a miniature receiver.
The primary task of a radio receiver is to separate the desired signal from a plurality of other signals in a common electromagnetic spectrum, both in-band and out-of-band, whose signal strengths range over many orders of magnitude. The task of separating these signals which differ in frequency are typically accomplished through frequency selective filters.
The task of separating the signals is twofold. First, the receiver's frequency of sensitivity must be tuned to that of the desired channel signal. Second, the bandwidth of sensitivity of the receiver must be restricted to that of only the desired channel signal while rejecting all frequencies outside of this bandwidth, thus separating the desired channel signal from all other signals.
Typically, these two tasks have been implemented separately. In conventional systems, the channel bandwidth is typically very narrow, perhaps 3 kHz or 30 kHz for a telephone quality voice channel. By comparison, carrier frequencies may range from tens of MHz to several GHz. To effect this selectivity at carrier frequency, the receiver typically utilizes a filter bandwidth in the order of 0.01% or less of the carrier frequency. Such narrow bandwidths require filter element quality factors (Q) in excess of 10,000, which become increasingly more difficult to achieve as carrier frequency increases. Such narrow bandwidths also require holding the accuracy and stability of the filter center frequency to comparable precision. This is compounded when the filter must be tunable.
In response to the aforementioned difficulties, super-heterodyne has been implemented to address the aforementioned difficulties. In a super-heterodyne receiver, the high radio frequency (RF) is converted to a lower, (super-audio) intermediate frequency (IF), of which the channel bandwidth is of a much greater proportion. For example, a 30 kHz channel at the fairly common IF of 10.7 MHz would require a Q of 350, which is readily achievable at this frequency.
In a heterodyne system, tuning is controlled by the local oscillator (LO). Selectivity, or reception bandwidth, is defined by the IF filter characteristic and is typically narrow. The frequency window of the IF filter characteristic is made to slide across the RF band. The center frequency of reception sensitivity is defined by the LO and the IF. Holding the combined accuracies of the local oscillator frequency and the center frequency of the IF filter is more readily achieved as compared to holding the accuracy of the center frequency of a narrow band, high frequency filter alone. Maintaining fixed reception bandwidth as the tuning frequency scans the RF band has become desirable.
Further, a filter is required in the RF amplifier ahead of the frequency conversion stage so as to reject the unwanted image frequency which is inherent in the basic heterodyning process. It is also advantageous to optimize the amplifier gain in the desired frequency range and more importantly, restrict the total power that the RF amplifier must handle. At a carrier frequency in the range of 250 MHz, an RF filter with a 2 MHz bandwidth, for example, would need a Q of only 125. If tracking of the RF filter center frequency is required, this precision also may be proportionately relaxed. To achieve an RF bandwidth of 2 MHz at 2 GHz carrier, however, would still require a fairly high Q of about 1000. This is one reason wide-tuning-range, high-frequency receivers typically employ double and triple conversion heterodyne systems.
Further, the implementation of the aforementioned RF filters would typically require strip-line, ceramic cavity, acoustic resonant, surface-acoustic-wave (SAW) devices or conventional inductors and accompanying capacitors. The aforementioned components require a significant space and have a substantial associated cost. Moreover, some of these components may require hermetic packages.
Another associated disadvantage is that, in view of the small physical space which is available in a miniature radio receiver, only a restricted energy source may be implemented therein. Therefore, the components of such a radio receiver must have a low power consumption in view of the restricted energy source. Further, the aforementioned dielectric based micro-strip and strip-line filters typically operate in an environment of the order of 50.OMEGA. impedance or less, thus requiring a large supply current from the aforementioned small restricted energy source.